Although it is well established that the speed of many complex behaviors is subject to priming, i. e. facilitation by previous responses, and once primed the increased response speed can persist for a considerable period of time, there is a dearth of information about the neuronal mechanisms that are responsible for the development of priming or for the memory component of the primed state. The long range goal of the research presented in this proposal is to understand the neural mechanisms that lead to the progressive increase of the speed of responding and of the mechanisms that are responsible for the memory component of this process, i.e. its persistence. We propose to use the well characterized feeding behavior and its neural substrate in Aplysia in order to extract the principles which relate the properties of the stimuli, synaptic plasticity and plasticity of the biophysical characteristics of behavior generating neurons to the process of the development and memory of the primed state. We propose to test a hypothesis that the development and memory of the primed state are a result of a series of steps that include: a) suppression of antagonistic responses, b) short term synaptic facilitation, and c) modulation of the biophysical characteristics of neurons by peptidergic co-transmitters that are released from the neurons that participate in the generation of the behavior. The proposed projects include: 1) characterization of synaptic plasticity of interneurons that facilitate antagonistic behaviors; 2) identification of cellular correlates of the development and of the memory of the primed state; 3) identification of transmitters and modulators contained in neurons that are involved in the development and memory of the primed state, 4) determination of the cellular mechanisms of action of these modulators and their contribution to the development and memory of priming. The research will utilize an interdisciplinary approach in which classical electrophysiological approaches are combined with cell biological, molecular and biochemical techniques to critically test the proposed hypotheses. The general importance of this type of research is likely to extend beyond the specific findings that we will obtain, as the mechanisms that are responsible for response selection, response priming, and the short term memory of these processes are likely to participate in a multitude of behaviors that for their efficiency rely on short term memory and selective attention.